Electric valve protective system



g- 2, 1939- M. IY-IURLIMANN 2,170,499

ELECTRIC VALVE PROTECTIVE SYSTEM Filed March 9, 1935 2 Shets-Sheet 1Aug. 22, 1939. M. HURLIMANN ELECTRIC VALVE PROTECTIVE SYSTEM Filed March9, 1935 2 Sheets-Sheet 2 Patented Aug. 22, 1939 ELECTRIC VALVEPROTECTIVE SYSTEM Max Hiirlimann, Baden, Switzerland, 'assignor toAktiengesellschaft Brown Bovcrl & Cie., Baden, Switzerland, ajoint-stock company of Switzerland Application March 9, 1935, Serial No.10,163 In Switzerland March 29, 1934 17 Claims.

This invention relates in general to improvements in electric valveprotective systems, and .more particularly to means for interrupting theflow. of current through an electric valve utilized in a direct currentinverting system upon occurrence of a disturbance in the operationthereof.

In direct current inverting systems utilizing an electric valve, thedirect current line supplying current to the alternating current outputline through the valve becomes, substantially in effect, short circuitedthrough the valve upon occurrence of a short circuit in the alternatingcurrent line. If the inverting system utilizes a plurality of valves;Short circuit may also result from irregularities in the sequence ofoperation of the valves. Such irregularities maybe caused by faultyoperation of the control electrodes of the valves, or by increase of theflow of current through the valves beyond the so-called load mamathereof as a result of an overload in the alternating current line or ofan abnormal decrease of the voltage-thereof. The flow of currentresulting from such short circuit is a continuous flow through one ofthe valves and, if

25 the valves are of the discontinuously controllable type, such flow ofcurrent cannot be interrupted by the action of the control electrodesthereof.

- Such interruption is then preferably obtained by discharge of acapacitor through an additional 30 valve connected thereby across thedirect current supply line.

It is therefore one of the objects of the present invention to provide aprotective system for an electric valve utilized for inverting directcur- 35 rent by which the flow of current through the valve may berapidly interrupted.

Another object of the present invention is to provide a protectivesystem for an electric valve utilized for inverting direct current bywhich '40 the flow of current from the supply'line may be diverted fromthe valve by discharge of 2. capacitor.

Another object of the present invention is to provide a protectivesystem for an electric valve 45 utilized for inverting direct current bywhich the valve may be rendered non-conductive upon interrnption of theflow of current 'therethrough.

Objects and advantages other than those above set forth will be apparentfrom the following de- 50 scripti'on when read in connection with theaccompanying drawings, in which:

Fig. l diagrammatically illustrates one embodiment of the presentinvention in which the operation of the protective system is controlledby 56 means of auxiliary electric valves; and

Fig. 2 is a diagram of some of the currents and voltages present in thecircuits of the embodiment illustrated in Figure l.

Referring more particularly to the drawings by characters of reference,reference numeral 6 rs designates the positive conductor of a directcur-- rent supply line energized from a suitable generator (not shown)and having a negative conductor I. Line 6, I supplies direct current tobe inverted into alternating current by the trans- 10 lating systemhereinafter described, such inverted current being supplied to analternating current output line 8 the frequency and voltage of which aredetermined by suitable means such as a synchronous generator (not shown)connected there- 15 with, as is well known in the art. The translat ingsystem comprises an electric valve or, preferably, a plurality ofelectric valves of any suitable type. Each valve comprises an anode 9with an associated control electrode ll; each valvemay be arranged in aseparate valve structure having a cathode, but the valves are preferablyall arranged within a single structure or so-called rectifier l2provided with a cathode i3 common to all valves and connected withconductor 1.

' Cathode I3 is provided with suitable means for bringing andmaintaining such cathode in current conductive condition as iswell knownin the art. Rectifier l 2 is herein illustrated as being of thediscontinuously controllable type in which the flow of current cannot beinterrupted by the action of the control electrodes thereof.

Each anode such as anode 9 is connected with one terminal of aninductive winding ll having another terminal connected with conductor 6,

- each winding thus being connected across line 6, l

by the associated valve or valve path. ihe several windings llpreferably constitute the primary winding structure of a polyphasetransformer if having a polyphase winding l6 connected with line 8. Inaddition to the above mentioned valves, rectifier l2 also comprisesanother valve consisting of an anode H with an associated controlelectrode l8 and the common cathode ll. Such valve connects a capacitorI9 across line 6, T in parallel with the inductive windings i4.Capacitor l9 may be gradually charged and maintained in chargedcondition by suitable means, such as a transformer 2| energized fromline 8 and having the secondary winding there-- of connected acrosscapacitor I! through an auxiliary electric valve 22 and a resistor 23.Capacitor i9 is charged at avoltage which is preferably of a valueseveral times greater than the value of the voltage or lines 6, i. Forthe proper g operation of capacitor I9, line 6, I must have aninductance of high values compared to the inductance of the inductivewindings I4, and such line may accordingly be provided with a reactor24.

The operation of the translating system consisting of rectifier I2 andtransformer I6 is controlled by suitable means for normally energizingthe control electrodes II to cause the valves 9, I3 to becomeintermittently and sequentially conductive. Such means preferablyinclude a transformer 26 energized from line 8 and having secondarywinding portions 21 severally connected with the control electrodes IIthrough resistors 28. The common point of such winding portions isconnected with cathode I3 through the taps of a voltage divider 29, uponwhich a unidirectional voltage is impressed from line 8 throughtransformer 3| and through auxiliary valves 32, such voltage beingrendered substantially uniform by the action of a capacitor 33. Controlelectrode I8 is connected with a tap of voltage divider 29 through aresistor 34. The positive terminal of voltage divider 29 is connectedwith cathode I3 through the primary windings of two control transformers36 and 31, through the coil and the contacts of a time delay relay 38,through an auxiliary discontinuously controllable valve 39 which iscontrolled through a shunt 4I inserted in conductor 1.. Shunt 4| may besubstantially non-inductive, but is preferably made inductive asdiagrammatically illustrated in the drawings. Valve 39 is provided witha control electrode connected with a suitable tap of voltage divider 29.The secondary Winding of transformer 31 is inserted in the connection ofcontrol electrode I8 with voltage divider 29 and may be bridged'by asurge limiting resistor 40. The secondary winding of transformer 36 iscohnected between the negative terminal of voltage divider 29 and thecontrol electrode of an auxil-- .iary discontinuously controllable valve42 connecting the common point of windings 21 with voltage divider 29through the coil and the contacts of a time delay relay 43.

In normal operation, lines 6, I and 8 being energized, voltage divider29 and transformer 26 cooperate to impress superimposed unidirectionaland alternating voltages between cathode l3 and control electrodes II,thereby causing each control electrode to be brought to superimposedunidirectional and alternating potentials with re spect to the potentialof cathode I3 taken as datum. Each control electrode II releases theflow of current through the associated anode upon reaching a potentialmore positive than the so-called critical potential, which will beassumed to be equal to cathode potential for the purpose of simplifyingthe explanation of the operation of the system. The flow of currentthrough each anode 9 is released by the associated control electrode IIwhen the potential of the anode follows the falling portion of the wavethereof but is more positive than the potential of cathode I3. In thediagram illustrated in Figure 2, the potential of conductor 1 isrepresented by line 44, the potential of conductor 6 being representedby line 46 distant therefrom by an ordinate AB representing the voltageof the direct current source connected with the line. The potential of aparticular one of anodes 9, of which the operation will be moreparticularly considered, may then be represented by a sine curve 41having line 46 as its axis. The flow of current is sequentially releasedthrough the several anodes to cause the potential of cathode I3 tofollow an undulating line 48 constituted of successive portions of sinewaves as is well known.

.During such operation, the flow of current through each anode isinterrupted as a result of the release of the flow of current throughthe following anode, which is then at a higherpotential, and areestablishment of such flow of current through the anode beingconsidered must be prevented until such anode returns to the properpotential. As each anode 9 is at positive potentials during the majorportion of the inoperative period thereof, transformer 26 and voltagedivider 29 must be so adjusted that each control electrode not onlyassumes a positive potential at a proper point of the cycle to releasethe flow of current through the associated anode, but must also retainsuch potential for a period of time not longer than the operating"period of the anode. The flow of current through cathode I3 is then asubstantially uniform current which may be represented by line 49 inFigure 2. Valve I'I, I3 is then maintained non-conductive by controlelectrode I8 which is continuously,

maintained negative by voltage divider 29. Capacitor I9 may therefore becharged from line 8 through transformer 2|, valve 22 and resistor 23 andthe voltage across the capacitor remains substantially constant at avalue represented by line 5I in Figure 2. The control electrodes ofvalves 39 and 42 are continuously maintained negative with respect tothe associated cathodes, and such valves therefore are maintainednonconductive.

To illustrate the operation of the protective system, it will be assumedthat the anode 9 being considered carries current, under a voltagerepresented principally by curve 41, during a time interval, CD which isthe normal operating interval thereof. rupted by transfer of the flow ofcurrent to the following anode, the anode 9 considered again becomingpositive with respect to cathode I3 at a time represented by point E. Itwill also-b e assumed that anode 9 again carries current at point E as aresult of the failure of the associated control electrode II to thenmaintain the valve 9, I3 considered in non-conductive condition. Fromtime E on, the potential of cathode I3 accordingly substantially followscurve 41, as such cathode is then substantially at the potential ofanode 9. The electromotive force opposed by the associated winding I4 tothe voltage of line B, I to limit the flow of current therethrough,

and which normally has an average effective value substantiallyrepresented by ordinate AB, then gradually decreases to reach the valuezero at time J. Valve 9, I3 and winding I4 then, in effect, constitute ashort circuit for line 6, 'I, and the voltage of winding I4 thereaftereven adds to the voltage of line 6, I instead of being opposed thereto.

The net voltage causing the flow of current through valve 9, I3 is thenrepresented by curve. 41 when read with respect to line 44, and suchvoltage is impressed mainly across reactor 24 which constitutes themajor portion of the impedance of the circuit. The flow of currenttherethrough accordingly increases from the value represented by line49, thereby causing the terminal voltage of shunt M to increase inproportion to the flow of current if the shunt is principally resistive,and also in relation with the rate of change of the flow of currenttherethrough if the shunt is inductive.

The connections of valve 39 are so adjusted that such increase in thevoltage across shunt 4i .depresses the potentialof the cathode of tube39 below thepotential of the associated control electrode, thusrendering valve 39 conductive at a time such as time F. A flow ofcurrent of predetermined value is thus suddenly initiated from voltagedivider 29 through the primary windings of transformers 36 and 31, thecoil of relay 39, valve 39 and shunt 4| back to voltage divider 29. Suchsudden initiation of the flow of current through the primary winding oftransformer 36 causes the appearance in the secondary winding thereof ofa positive voltage impulse of suflicient magnitude to momentarily renderthe control electrode of valve 42 positive with respect to theassociated cathode. 'Valve 42 accordingly becomes conductive and, byshort circuiting a portion of voltage divider 29, causes the potentialof the common point of windings 21 to become more negative to an extentsuch as to continuously maintain the control electrodes II at a negativepotential, thereby preventing the further transfer of the discharge fromone of anodes 9 to another, and also preventing the reestablishment offlow of current through rectifier [2 after interruption thereof by theprocess set forth hereinafter.

A positive voltage impulse also appears in the secondary winding oftransformer 37, such impulse being of magnitude suilicient to'causeelectrode l8 to become momentarily positive. Valve l1, I3 is accordinglymade conductive and capacitor I9 is discharged therethrough into line 6,I. Cathode I3 is thereby brought substantially instantly to a potentialrepresented by point G, of which the ordinate GF is equal to theordinate of line 5|, the voltage then impressed across reactor 24 beingrepresented by ordinate GH. The flow of current through anode I! may berepresented by a curve 52, such flow of current tending to becomeoscillating as a result of the serial connection of capacitor [9 withreactor 24, but being limited to a single unidirectional impulse by theaction of valve l1, l3.

Upon valve l1, l3 being made conductive, the current then flowingthrough cathode I3 from anode 9 is diverted from such anode and startsto flow instead through anode ll. Although cathode I3 is thereby broughtto the potential represented by point G, which is higher than thepotential of anode 9 represented by point K. The flow of current throughanode 9 is temporarily maintained by the magnetic energy stored in thecore of winding l4 and rapidly falls to zero as is indicated by curve53. Such decay of the anode current occurs as if the discharge ofcapacitor I 9 were actually sending, through anode 9 and winding I4, areverse current component, which becomes of considerably greatermagnitude than the current sent by the capacitor through cathode l3,reactor 24 and line 6, l as a result of the large inductance of thereactor compared to that of winding l4. The flow of current throughvalve 9, l3 having ceased, such flow of current is not reestablishedtherethrough for the reason that control electrode I l is then alreadymade negative by the operation of valve 42. The flow of current throughanode l1 continues even after capacitor I9 is completely discharged, toreturn to the capacitor the energy stored in the core of reactor 24, thecapacitor being thereby recharged at a voltage somewhat less inmagnitude than the initial voltage thereof and of opposite polarity.During such discharge and reverse charge of capacitor I 9, the voltagethereof follows curve 54 and the potential of cathode ll follows curveGML.

The flow of current through capacitor l9 then stops as a result of theaction of valve l1, l3, and the potential of cathode I3 is then broughtto the potential of conductor 1 and maintained at such potential by thesource connected with line 6, I. The course of the potential of cathodel3 during the above described sequence of operations is represented inheavy line in Fig. 2. During such sequence of operations, transformer 2|and valve 22 continue to supply current to capacitor l9 and, uponinterruption of the flow of current through valve ll, I3, graduallyrecharge the capacitor under a voltage following curve 56. Such chargingcontinues even when anode H becomes positive with respect to cathode l3,valve l1, l3 being continuously maintained non-conductive by controlelectrode [8 in the absence of positive impulses in the secondarywinding cc transformer 3".

It will be observed that, during the above sequence of operations, thecurrent flowing through cathode I3 is the sum of the current throughWinding l4 and through capacitor l9, and may therefore be represented bya curve 51 having ordinates equal to the sum of ordinates of curves 49,52 and 53, Such current reaches a maximum value when the cathodepotential reaches the potential of conductor 1 at time M. In the absenceof valve 42, the flow of current would then reestablish itself throughthe valves 9, l3 of rectifier l2, and the translating system wouldautomatically return to the normal operation thereof, except that, as aresult of the inductance of reactor 24, such flow of current wouldinitially tend to maintain the cathode current at the value reachedthereby at time M.

The elements of the translating system may be so dimensioned that suchflow of current then does not exceed the so-called load limit thereof inthat even then each valve of the rectifier, after carrying current,transfers its current to the following valve and thereafter becomesdeionized and is made non-conductive while still M exceeds the loadlimit of the translating system, and resumption of normal operation atsuch time would generally result in the occurrence of another shortcircuit through one of the valves. Valve 42 is therefore utilized forrendering all valves 9, l3 non-conductive at time F, except that thevalve then carrying current becomes nonconductive upon interruption ofthe flow of current therethrough at time P. During the interruptingoperation, the flow of current through the primary winding oftransformer 31 is of constant value, and such transformer is thereforewithout further action on the potential of control electrode 3, which isagain maintained at a negative potential by means of voltage divider 29.Valve l1, l3 accordingly becomes non-conductive immediately uponcessation of the flow of current therethrough, thereby permittingrecharging of capacitor l9.

After a predetermined time delay, relay 38 opens the contacts thereof tocause valve 39 to again become non-conductive, such operation causingthe appearance, in the secondary wind- Cal ings of transformers 36, 31,of voltage impulses of such polarity as not to affect the operation ofthe system. Relay 43 thereafter also opens the contacts thereof, therebyreturning valve 42 to the non-conductive condition and causing theunidirectional potential of control electrodes II to return to thenormal value thereof to reestablish the normal operation of rectifierl2. Relay 43 is preferably so adjusted as to operate only aftercapacitor I9 is again substantially completely charged so that thecapacitor may already operate to extinguish any short circuits inrectifier I! immediately upon reestablishment of the flow of current tothe rectifier.

It will be apparent that, under favorable circumstances, the circuits ofthe system may be so arranged that control electrode l8 renders valvell, l3 conductive before the time J at which the voltage of winding I4reverses. If capacitor i9 is 'not then charged, anode I! is at thepotential of conductor 6 represented by line 46 in Fig. 2 and istherefore at a higher potential than the anode 9 considered. The flow ofcurrent thruogh anode 9 may then again be diverted to anode I! if theelectrical characteristics of the circuits are within suitable ranges ofvalues. The charging means for capacitor l9 may then be omitted, but ingeneral such charging means are preferably retained to insure operationof the protective system in the manner above described.

' Although but one embodiment of the present invention has beendescribed and illustrated, it will be apparent to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the spirit of the invention or from the scope of theappended claims.

.It is claimed and desired to secure by Letters Patent:

1. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the flowof current therethrough to said alternating current line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, and means for causing interruption of the flow ofcurrent through the first said valve including means for rendering thesecond said valve conductive.

2. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the flowof current theretli'rough to said alternatingcurrent line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, means for causing interruption of the said flow ofcurrent through the first said valve including means for causingdischarge of the said capacitor through the second said valve, and meansfor preventing the reestablishment of the flow of current through thefirst said valve.

3. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the flowof current therethrough to said alternating current line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, and

. means responsive to an operative condition of said system for causingsaid capacitor to discharge through the second said valve.

4. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the flowof current therethrough to said alternating current line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, and means for causing interruption of the said flowof current through the first said valve including means operable uponthe occurrence of substantial short circuiting of said direct currentline through the first said valve to render the second said valveconductive.

5. An electric current inverting system comprising a direct currentsupply line, an altemating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the fiowof current therethrough to said alternating current line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, means for charging said capacitor, and means forcausing interruption of the said fiow of current through the first saidvalve and including means for rendering the second said valveconductive.

6. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the fiowof current therethrough to said alternating current line, anotherelectric valve, a capacitor'connecting the second said valve across saiddirect current line, means for rendering the first said valveintermittently conductive to control the flow of current to saidalternating current line, means for normally maintaining the second saidvalve non-conductive, and means for causing interruption of the saidflow of current through the first said valve and including means forrendering the second said valve conductive.

7. An electric current inverting system comprising a direct currentsupply line, an altemating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the flowof current therethrough to said alternating current line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, means for rendering the first said valveintermittently conductive to control the said fiow of current, means fornormally maintaining the second said valve non-conductive, means forcausing interruption of the said flow of current through the first saidvalve incuding means for rendering the second said valve conductive, andmeans for rendering the second said valve non-conductive uponinterruption of the flow of curren therethrough.

8. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the flowof current therethro'ugh to said alternating current line and having acontrol electrode for controlling the operation thereof, a capacitor,means for gradually charging said capacitor, another electric valveconnecting said capacitor across said direct current line and having acontrol electrode, means for normally maintaining the second saidcontrol electrode at a potential for maintaining the second said valvenon-conductive, means for normally energizing the first said controlelectrode at potentials rendering thefirst said valve intermittentlyconductive, and means for causing interruption of the said flow ofcurrent through the first said valve including means for varying thepotential of the second said control electrode to render the second saidvalve conductive.

9. In an electric translating systemja direct current supply line, analternating current output line, an inductive winding connected withsaid alternating current line, an electric valve connecting the saidwinding across said direct current line for the exchange of currentbetween said lines and having a control electrode for controlling theoperation thereof, a capacitor, means for gradually charging saidcapacitor, another electric valve connecting said capacitor across saiddirect current line and having a control electrode, means fornormallymaintaining the second said valve non-conductive, means for normallyenergizing the first said control electrode at potentials rendering thefirst said valve intermittently conductive, means for causinginterruption of the flow of current through the first said valveincluding means for varying the potential of the second said controlelectrode to render the second said valve conductive, and means forvarying the potential of the first said control electrode to render thefirst said valve non-conductive upon interruption of the flow of currenttherethrough.

10. In an electric translating system, a direct current supply line, analternating current output line, an inductive winding connected withsaid alternating current line, an electric valve connecting the saidwinding across said direct current line for the exchange of currentbetween said lines and having a control electrode for controlling theoperation thereof, a capacitor, means for gradually charging saidcapacitor, another electric valve connecting said capacitor across saiddirect current line and having a control electrode, means for normallymaintaining the second said control electrode at a potential maintainingthe second said valve non-conductive, means for impressing superimposedunidirectional and alternating potentials on the first said controlelectrode to render the first said valve intermittently conductive,means for causing interruption of the flow of current through the firstsaid valve including means for varying the potential of the second saidcontrol electrode to render the second said valve conductive, and meansfor giving a negative increment to the said unidirectional potential torender the first means for causing interruption of the said flow ofcurrent through the first said valve including means responsive to anoperating condition of said system for causing said capacitor todischarge through the second said valve and through said direct currentline, and means comprising a reactor for limiting the rate of dischargeof said capacitor through said direct current line.

12. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current linefor the flowof current therethrough to said alternating current line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, means for charging said capacitor, means forcausing interruption of the said flow of current through the first saidvalve including means for rendering the second said valve conductive,and a reactor in said direct current line to oppose variations in themagnitude of the flow of current therein.

13. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, a plurality ofinductive windings connected with said alternating current line, aplurality of electric valves severally connecting said windings acrosssaid direct current line for the flow of current therethrough to saidalternating current line, a capacitor, means for maintaining saidcapacitor in charged condition, another electric valve connecting saidcapacitor across said direct current line, and means for causinginterruption of the said flow of current through said plurality ofvalves including means for rendering the second said valve conductive.

14. An electric current inverting system com-' prising a direct currentsupply line, an alter-- nating current output line, a plurality ofinductive windings connected with said alternating current line, aplurality of electric valves severally connecting said windings acrosssaid direct current line for the flow of current therethrough to saidalternating current line, a capacitor, means for maintaining saidcapacitor in charged condition, another electric valve connecting saidcapacitor across said direct current line, means for causinginterruption of the said flow of current through said plurality ofvalves including means for rendering the secondsaid valve conductive,and means for rendering said plurality of valves non-conductive uponcessation of the flow of current therethrough.

15. An electric current inverting system col prising a direct currentsupply line, an alternating current output line, a plurality ofinductive windings connected with said alternating current line, aplurality of electric valves severally connecting said windings acrosssaid direct current line for the flow of current therethrough to saidalternating current line, 2. capacitor, means for maintaining saidcapacitor in charged condition, another electric valve connecting saidcapacitor across said direct current line, means for causinginterruption of the said flow of current through said plurality ofvalves including means for rendering the second said valve conductive,and means for rendering said plurality of valves non-conductive uponcessation of the flow of current therethrough, and a reactor in saiddirect current line to limit the rate of discharge of said capacitortherethrough.

16. An electric current inverting system comprising a direct currentsupply line, an alternating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the fiowof current therethrough to said alternating current line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, means for rendering the first said valveintermitently conductive to control the said fiow' of currenttherethrough, means for normally maintaining the second said valvenon-conductive, means for causing interruption of the said fiow ofcurrent through the first said valve including means for rendering thesecond said valve conductive, and means for rendering the first saidvalve non-conductive upon interruption 01 the flow of currenttherethrough.

17. An electric current inverting system comprising a direct currentsupply line, an altemating current output line, an inductive windingconnected with said alternating current line, an electric valveconnecting the said winding across said direct current line for the flow01 current therethrough to said alternating current line, anotherelectric valve, a capacitor connecting the second said valve across saiddirect current line, means for rendering the first said valve.

